In the wireless communications industry, a premium is placed on the ability to amplify wide bandwidth signals, e.g., spread spectrum signals, in a highly efficient manner. As an example, a typical eighteen-channel base station requires approximately 540 watts of RF power output (30 watts per each channel). Assuming a typical power amplifier efficiency of 5 percent, the amount of power needed to generate an RF power output of 540 watts will be 10.8 kW, with 10.26 kW being dissipated as heat. This dissipated heat represents a drawback in that it not only requires the use of fans and heat sinks to cool the base station, but also translates to wasted energy, thereby reducing battery life. In short, the cost of the base station increases as the efficiency of the power amplifiers used in the base station decreases.
Although various attempts have been made to address this problem, it remains difficult to design a high efficiency power amplifier that is able to linearly amplify wide bandwidth signals. This is due to the paradoxical nature of a typical amplifier, which exhibits a wide bandwidth capability that is inversely proportional to its efficiency. The amplification of spread spectrum signals, such as code division multiple access (CDMA) signals, which typically have high peak-to-average signal amplitude ratios, make it difficult, if not impossible, to continuously operate the power amplifier in saturation, thereby reducing the efficiency of a power amplifier even further.
A method that has been proposed to solve this problem involves the use of envelope elimination and restoration (EER), which is a technique through which highly efficient radio frequency (RF) power amplifiers can be combined to produce a high efficiency linear amplifier system. In this method, a modulated input signal is split into two paths: an amplitude path through which the envelope of the modulated input signal is processed, and a phase path through which the phase modulated carrier of the modulated input signal is processed. The envelope of the modulated input signal is amplified through a highly efficient amplifier, which operates at the narrower modulated bandwidth, i.e., the bandwidth of the envelope, thereby producing an amplified envelope signal. A high frequency, highly efficient amplifier is then used to modulate the high frequency phase modulated carrier with the amplified envelope signal, thereby generating an amplified replica of the modulated input signal. Specifically, the amplifier that generates the amplified envelope signal acts as the DC power supply to the high frequency amplifier. The efficiency of this EER amplifier system can be calculated by multiplying the efficiencies of the two amplifiers. For example, if the efficiency of each of the amplifiers is 50%, the total efficiency of the EER amplifier system will be 25%.
Although the use of an EER amplifier system to amplify wide bandwidth modulated signals is, in general, beneficial, its efficiency and maximum modulation bandwidth is dependent upon the efficiency and bandwidth of the power supply amplifier.
Accordingly, it would be desirable to provide apparatus and methods for increasing the efficiency and bandwidth of an amplifier for a variety of purposes, which may include the provision of DC power to an RF power amplifier within an EER amplifier system.